Coaxial cable termination arrangement

ABSTRACT

A termination arrangement for a coaxial cable comprises a dimensionally heat-recoverable sleeve that encloses a connecting element for the screen of the cable and a quantity of solder for forming a permanent electrical connection between the connecting element and the cable screen. The connecting element has a generally cylindrical portion whose diameter is capable of changing and which is held in a state of larger diameter by the solder and/or the sleeve, the element changing to a smaller diameter state when the sleeve is recovered. In use, the arrangement is positioned over a coaxial cable which has been prepared so as to expose an appropriate length of the central conductor, dielectric and screen. when the sleeve is recovered, the solder fuses and the connecting element contracts around the screen.

This invention relates to the formation of electrical connections tocoaxial cables and especially to the termination of the screens of suchcables.

In modern communication systems it is important that connections tocoaxial cables, for example connections between cables employing acoaxial connection, connections to printed circuit boards and the like,are impedance matched in order to minimize the amplitude of signalreflections. Clearly also the connections should be made with a lowinstalled cost. At present coaxial cables are usually terminated atcoaxial connectors in one of three ways: by crimping, by clamping or bymeans of solder. The solder arrangements generally exhibit the bestelectrical performance but are normally employed only for prototypeassembly in view of their cost, whereas the crimp type of terminationexhibits relatively poor electrical characteristics but is susceptibleto relatively high volume production. In addition, it is often notpossible to make crimp replacement terminations in the field. Cableclamp arrangements, on the other hand, can be employed in the field butare only susceptible to low volume production.

According to the present invention, there is provided a terminationarrangement for a coaxial cable, which comprises a dimensionallyheat-recoverable sleeve that encloses a connecting element for thescreen of the cable and a quantity of solder for forming a permanentelectrical connection between the connecting element and the cablescreen, the connecting element comprising a ferrule having a generallycylindrical portion which has one or more axial slits that allow itsdiameter to change and which is held in a state of larger diameter bythe solder and/or the sleeve, the element changing to a smaller diameterstate when the sleeve is recovered.

In order to terminate the screen of a coaxial cable, the cable jacket,screen and dielectric layer are each cut back in order to expose anappropriate length of the central conductor, dielectric and screen, thearrangement is positioned over the cable so that the exposed length ofthe cable screen is located within the generally cylindrical portion ofthe connecting element, and the assembly is briefly heated, for exampleby means of an infrared lamp or a hot-air gun, in order to cause thesleeve to recover, the solder to fuse and the connecting element tocontract around the cable screen. The arrangement according to theinvention has the advantage that it is relatively simple inconstruction, thereby allowing high volume production, but can form atermination with a high degree of impedance matching with the cable. Inaddition, it is possible to repair a cable screen termination with oneaccording to the invention in the field. As is well known, the degree towhich different parts of a coaxial line are impedance matched willdepend on the ratio of diameters of the screen and the central conductoras well as the relative permittivity of the dielectric. According to thepresent invention, the connecting element may be configured so that,when the termination arrangement is installed about a coaxial cable, theconnecting element will contract to a predetermined value but no more.Thus, the connecting element may be arranged so that it will contractenough to form a snug fit with the outer surface of the exposed screenwithout deforming the screen significantly and thereby affecting thecharacteristic impedance of that part of the cable. Although theconnecting element does not compress the cable screen enough to deformit significantly, i.e. enough to alter the impedance characteristics, agood electrical connection will be formed between the connecting elementand the cable screen due to the solder.

The arrangement may be employed in any of a number of configurations.For example the connecting element may be biassed to a state of smallerdiameter and be held in its state of larger diameter by the solderand/or by the sleeve so that when the arrangement is heated and thesolder fuses or the heat-recoverable sleeve material becomes taught theparts forming the connecting element will be allowed to move toward oneanother. Alternatively, the connecting element may be forced into itsstate of smaller diameter by recovery of the sleeve. Whichever form isemployed it is important that the distance through which the parts ofthe connecting element are allowed to move together is defined in ordernot to crush the coaxial cable. This may be achieved, for example byproviding the connecting element with one or more axial slits (or slots)that allow its diameter to change but will prevent further contractionof the connecting element once the slits have closed. The slits mayextend the entire length of the connecting element if desired. In thiscase, if more than one slit is present this will necessarily mean thatthe connecting element is formed in more than one part, each part beingsupported by the sleeve wall. Preferably, however, the connectingelement is formed in one piece which means that it will have only oneslit that extends the entire length of the element or that the slitsextend only partly along the element. In another form of arrangement theconnecting element (or at least the generally conical part of it) may beformed as a spring, e.g. a spring that is generally uniform along itsaxis and has a spiral cross-section, or a generally helicoidal spring,so that the element may contract by coiling up.

In the case of a connecting element in the form of a spring, it may beformed from sheet metal, e.g. spring steel, berylium copper or hardtempered copper or from wire (formed, for example, from the samematerials) which may have a circular or polygonal cross-section. In thecase of elements being provided with slits, the element may be machinedfrom a solid block of metal, e.g. brass, or it may be formed by pressingit out of a strip of metal. In the latter case it may be appropriate andsimplify handling of the arrangements if they are retained on the stripuntil use. Thus, the invention also provides an array of terminationarrangements according to the invention in which the connecting elementsof the termination arrangements have been formed on a strip of metal andare individually removable from the strip. Such an array may be employedmerely in order to simplify handling of the individual arrangementsintended to be located on separate coaxial cables or it may be employedsimultaneously to terminate a number of coaxial cables which form partof a composite cable, by means of automatic or semiautomatic wirehandling equipment.

Although it will normally be necessary for a termination to be designedto fit one size of cable only, a single arrangement may be designed tofit several different types of connector systems, for example coaxialconnectors including coaxial contacts, and bulkhead feedthroughs, andterminations to printed circuit boards. Thus, according to yet anotheraspect, the invention provides a connector for a coaxial cable, whichcomprises a male or ferrule contact for the central conductor of thecable, a termination arrangement according to the invention forterminating the screen of the cable, and a connecting piece which holdsthe termination arrangement and the central conductor contact, theconnecting piece providing screen continuity for the cable.

As stated above, the sleeve is dimensionally heat-recoverable, that isto say the article has a dimensional configuration that may be madesubstantially to change when subjected to heat treatment.

Usually these articles recover, on heating, towards an original shapefrom which they have previously been deformed but the term"heat-recoverable", as used herein, also includes an article which, onheating, adopts a new configuration, even if it has not been previouslydeformed.

In their most common form, such articles comprise a heat-shrinkablesleeve made from a polymeric material exhibiting the property of elasticor plastic memory as described, for example, in U.S. Pat. Nos.2,027,962; 3,086,242 and 3,597,372. As is made clear in, for example,U.S. Pat. No. 2,027,962, the original dimensionally heat-stable form maybe a transient form in a continuous process in which, for example, anextruded tube is expanded, whilst hot, to a dimensionally heat-unstableform but, in other applications, a preformed dimensionally heat-stablearticle is deformed to a dimensionally heat-unstable form in a separatestate.

In the production of heat-recoverable articles, the polymeric materialmay be cross-linked at any stage in the production of the article thatwill enhance the desired dimensional recoverability. One manner ofproducing a heat-recoverable article comprises shaping the polymericmaterial into the desired heat-stable form, subsequently cross-linkingthe polymeric material, heating the article to a temperature above thecrystalline melting point or, for amorphous materials the softeningpoint, as the case may be, of the polymer, deforming the article andcooling the article whilst in the deformed state so that the deformedstate of the article is retained. In use, since the deformed state ofthe article is heat-unstable, application of heat will cause the articleto assume its original heat-stable shape.

Any material to which the property of dimensional recoverability may beimparted may be used to form the sleeve. Preferred materials includelow, medium or high density polyethylene, ethylene copolymers, e.g. withalpha olefins such as 1-butene or 1-hexene, or vinyl acetate, polyamidesor fluoropolymers, e.g. polytetrafluoroethylene, vinylidine fluoride orethylene-tetrafluoroethylene copolymer.

Also, as mentioned above, the arrangement includes a quantity of solder,i.e. a quantity of soft solder as distinct from brazing material, forforming a permanent solder connection. The solder may, for example,simply be in the form of an Sn₆₃ Pb₃₇ eutectic composition which willmelt as the device is heated and the sleeve recovers, or more than onesolder composition having differing melting points may be employed, asdescribed in International Application No. WO88/09068. In this form ofdevice, melting of the higher melting point component, e.g. Sn₉₆.5 Ag₃.5eutectic will provide a visual indication that the device has beenheated sufficiently to melt the lower melting point composition and toform a satisfactory solder joint. If desired the lower melting pointsolder may be a non-eutectic composition and, for example as describedin International Application No. PCT/GB90/00234, the higher and lowermelting point solder compositions may together form a eutecticcomposition. For example, a non-eutectic Sn₆₀ Pb₄₀ lower melting pointcomponent may be employed with a higher melting point component formedfrom pure tin in relative amounts that an Sn₆₃ Pb₃₇ eutectic is formed.The disclosures of these two patent applications are incorporated hereinby reference. An advantage of employing a two component solder, andespecially a tin, Sn₆₀ Pb₄₀ combination is that it reduces thepossibility of "wicking" that is to say, travel of the solder along theconductors and away from the joint area due to capillary action by thestrands of the screen, which can be caused by prolonged heating of thedevice.

The solder may be positioned anywhere where it will be able to flow intothe connecting element to form a solder joint. The solder may beemployed in the form of a ring or in any other form for example a ball,and may be disposed symmetrically about the connecting element axis oroffset from it.

Several forms of termination arrangement in accordance with the presentinvention will now be described by way of example with reference to theaccompanying drawings, in which:

FIG. 1 is a perspective view of an array of termination arrangementsaccording to the invention located on a metal strip;

FIG. 2 is a side view showing one of the termination arrangements ofFIG. 1 together with a length of coaxial cable;

FIG. 3 is a perspective view of one of the termination assemblieslocated on a metal strip together with a length of coaxial cable and acontact terminal for the centre conductor of the cable;

FIG. 4 is a side view of a pcb connection according to the invention;

FIGS. 5 and 6 show further forms of termination arrangement according tothe invention; and

FIG. 7 shows yet another form of termination.

Referring to the accompanying drawings, FIG. 1 shows an array of devices1 for terminating the screens of coaxial cables according to the presentinvention located on a metal sheet 2. The devices 1, one of which isshown in FIG. 2, each comprises a heat-shrinkable transparent sleeve 3formed from radiation crosslinked polyvinylidine fluoride that has beenpartially shrunk onto a machined brass ferrule 4 and a solder ring 5that is located on the ferrule. The ferrule 4 is generally cyclindricaland has a flange 6 at one end thereof for location of the terminationassembly in a connector of the like. Also, the ferrule has a pair ofaxially extending slots 7 that extend from the other end of theconnecting element that is located inside the shrinkable sleeve 3 andextends about two thirds the length of the ferrule, so that the solderring 5 is located over the slots.

In order to terminate the screen of a coaxial cable 8, the jacket 9,screen 10, for example in the form of a braid, and dielectric 11 areeach cut back so that appropriate lengths of the central conductor 12,dielectric 11 and screen 10 are exposed. The coaxial cable is theninserted into the open end of the device until the central conductor 12and dielectric extend through the ferrule 4 and the screen is locatedwithin the ferrule. The assembly is then heated in order to recover thesleeve 3 about the cable and to fuse the solder ring 5. At the same timethe contraction of the sleeve 3 causes the opposed parts of the ferrule4 that are separated from one another by the slots 7 to move toward oneanother into contact with the cable screen. The fused solder 5 will forma good electrical contact between the cable screen 10 and the ferrule 4and remove any air gaps between them.

FIG. 3 shows the arrangement of FIG. 1 (in which only one terminationdevice 1 is shown) in combination with a separate array of devices 13(of which only one is shown) for forming a pin or male contact for thecentral conductor of the coaxial cable. Both arrays of devices aremounted on a strip 2 of metal, in this case the strips being formed fromthe same material from which the ferrule 4 of the screen terminationarray and the pin 14 of the central conductor termination array isformed, the ferrules 4 and the pins 14 normally being stamped out of thestrips 2. The pin contact devices 13 and the screen termination devicesare in register so that an approximately cut back coaxial cable 8 can belocated in both devices and both the central conductor and the cablescreen can be terminated simultaneously by heating both devices. Thecentral conductor terminating device 13 also includes a solder ring 16and is enclosed in a thin sleeve of heat-shrinkable polyvinylidinefluoride 15 so that, after heating, insulated solder joints are formedto both conductors.

This form of array may be employed to terminate coaxial cablesindividually or they may be used to terminate a number of cablessimultaneously, for example by means of automatic or semi-automaticequipment.

The coaxial cables terminated in this way may then be located in any ofa number of different coaxial cable connectors, for example a BNC, SMAor TNC connector, or the like, in which a conductive housing is employedto provide screen continuity between the two coaxial cables.

The central conductor need not, however, also be terminated. For examplethe connection of a coaxial cable to a printed circuit board 18 is shownin FIG. 4 in which the cable screen is terminated as above but thecentral conductor 12 and dielectric 11 are bent at 90° to the rest ofthe cable in order to be introduced into a through hole or via 19 of theboard. The cable screen is connected to a conductive track 20 on theboard via the ferrule 4 and a ferrule holding fixture 21 shownschematically while the central conductor 12 of the cable is soldered toa conductive lining 22 of the through hole 19.

FIG. 5 shows schematically one alternative form of terminationarrangement according to the invention in which the connecting elementor ferrule is formed in two halves 4' and 4" (this corresponding inprinciple to the ferrule of FIG. 1 in which the slots 7 extend theentire length of the ferrule. In this embodiment both ferrule halves 4'and 4" are held in the heat-shrinkable sleeve 3 separated from oneanother by axially extending indentations 25 in the solder ring 5, andoptionally also by axially extending indentations in the sleeve 3 whichextend the slot between the two halves 4' and 4" of the ferrule andprevent them moving toward one another. The indentations will normallybe formed by a punching operation. When the device is heated the solderring 5 fuses and the wall of the sleeve 3 becomes taught, therebyallowing the two ferrule halves 4' and 4" to move toward one anotherunder the recovery forces of the sleeve 3 as shown in FIG. 5c.

Another alternative form of device is shown in FIG. 6. In this devicethe ferrule 4 has a single slot 7 that extends along the entire lengthof the ferrule but does not divide it into two halves. The solder ring 5and optionally also the sleeve 3 may be provided with an axiallyextending indentation 25 similar to that shown in FIG. 5 which extendsinto the slot 7 and prevents it closing. In this embodiment the ferrule4 may be biased to close the slot 7, closing being prevented by thesolder indentation, so that when the device is heated duringinstallation the ferrule 4 will begin to contract in diameter under itsown influence as soon as the solder indentation 25 and any sleeveindentation has disappeared.

Yet another form of device is shown schematically in FIG. 7, in itsinitial form in FIG. 7a and in its recovered form in FIG. 7b. In thisform of device the connecting element has a helicoidal configuration andwill contract radially under the action of the heat-shrinkable sleeve 3or under its own action, when the assembly is heated. One end of theconnecting element or ferrule 4 has a slot 7 which closes as the elementcontracts radially and then prevents and substantial subsequentcontraction.

I claim:
 1. A termination arrangement for a coaxial cable, whichcomprises a dimensionally heat-recoverable sleeve that encloses aconnecting element for a screen of the cable and a quantity of solderfor forming a permanent electrical connection between the connectingelement and said screen, the connecting element (a) comprising a ferrulehaving a generally cylindrical portion which has one or more axial slitsthat allow its diameter to change and (b) being biased to a state ofrelatively smaller diameter and being held in a state of relativelylarger diameter by means of at least one of the solder and the sleevebeing located within part of said at least one of the slits, the elementchanging to a smaller diameter state when the sleeve is recovered.
 2. Anarrangement as claimed in claim 1, wherein the connecting element hasbeen formed in one piece.
 3. An array of termination arrangements, eachof which is as claimed in claim 1, in which the connecting elements ofthe termination arrangements have been formed on a strip of metal andare individually removable from the strip.
 4. A connector for a coaxialcable, which comprises a contact for the central conductor of the cable,a termination arrangement as claimed in claim 1 for terminating thescreen of the cable and a connecting piece which holds the terminationarrangement and the central conductor contact, the connecting pieceproviding screen continuity for the cable.